Portable mobile satellite communication has been receiving an attention as a next generation mobile communication, and many related technologies have been actively researched and developed. The portable mobile satellite communication uses a high frequency band, for example, higher than 10 GHz. Since the high frequency band is easily attenuated by wind or rain, various problems may be arisen. Especially, the portable mobile satellite communication requires a high design cost to sustain the receiving power level at an uplink similar to the receiving power level of a terrestrial wave because of the structural limitation of a portable terminal and the large power attenuation characteristic of satellite communication. Therefore, a synchronization unit of a portable mobile satellite MODEM must be designed to operate in a low signal-to-noise ratio (SNR).
Recently, an iterative phase synchronization method for a system using a turbo code at a low SNR was introduced. In this iterative phase synchronization method, a phase is estimated using the temporal discrimination variable of a symbol at every iteration stages, and the discrimination variable is updated by compensating the phase at a following iteration stage. As described above, the phase is continuously updated at iterative decoding stages unlike a conventional phase synchronization method. Therefore, an additional phase synchronization block is not required. Also, the iterative phase synchronization method uses phase error information obtained from one codeword only for decoding a corresponding code. Therefore, a delay problem caused by an unnecessary loop that is a feedback based synchronizing algorithm is not arisen in the iterative phase synchronization method.
However, since the iterative phase synchronization method assumes that a phase error is constant in a codeword, the phase errors of codewords are estimated based on the constant phase error at every iteration stages. Accordingly, the conventional iterative phase synchronization method cannot be used when a phase rotates in a codeword due to a residual frequency. Because the iterative phase synchronization method seriously degraded the performance thereof if the residual frequency error is present.
The residual frequency error is commonly generated in a codeword by a Doppler in the mobile communication environment. Although the Doppler is not present, the residual frequency error is generated when a preceding frequency synchronizing algorithm is not perfect. If the length of a codeword is comparatively short although the residual frequency error is generated, it is possible to assume a phase error as a constant in a codeword. However, since the length of a codeword having a high coding gain such as a turbo code or low-density parity-check (LDPC) is generally long, it is unreasonable to assume the phase error as the constant. As described above, to assume the phase error as a constant is very strict assumption. Therefore, the conventional iterative phase synchronization method has restricted applicability.
If code symbols are sequentially transmitted in a channel using a carrier wave, the iterative phase synchronization method can be applicable. On the contrary, since a high speed OFDM based transmission system modulates the code symbol stream of one codeword based on the OFDM modulation scheme, the code symbols are transmitted with different subcarrier waves as many as the number of the subcarrier waves. That is, the code symbols are grouped into N groups, where N denotes the number of subcarrier waves in the OFDM system. Then, the OFDM system transmits the code symbols sequentially in each subcarrier wave and transmits the code symbols with subcarrier waves in parallel. The subcarrier waves have different initial phase errors due to a timing error, a timing jitter and fading. It means that the phases of each subcarrier wave must be estimated independently. That is, a frequency and phase restoring method for a single carrier wave system must be modified to be suitable to the characteristics of the OFDM modulation method.
Recently, there have been many researches in progress for developing various wireless communication systems using an OFDM based wideband transmission method according to the requests of high speed data transmission. Also, the OFDM modulation scheme has been examined to apply it to the personal mobile satellite communication. According to such trends, there is a demand for a low power synchronization method for the OFDM modulation scheme.